Recently, as backlight applications of liquid crystal displays and the like, studies are intensively concentrate on a rare gas discharge lamp having an external electrode operated with dielectric barrier discharge. This is because the rare gas discharge lamp dose not need mercury and light emission efficiency is not changed by change of mercury vapor pressure, and also because it is preferred from the environmental point of view. In lighting operation using dielectric barrier discharge, the dielectric layer is charged by application of a driving voltage, and discharge is induced by a high voltage generated when the driving voltage is inverted, and hence a rectangular wave voltage of high frequency is used as driving voltage.
Generally, a safety circuit in case of emergency is included in the lighting apparatus of almost all discharge lamps, not limited to dielectric barrier discharge lamps. This is intended to prevent inconvenience, or breakage of a ballast circuit due to continued application of a high voltage to a discharge lamp, or shortening of life due to abnormal high temperature, when the lamp fails to light due to leak of discharge tube of a discharge lamp.
An example of a safety circuit of a dielectric barrier discharge lamp lighting apparatus is disclosed in patent document 1.
FIG. 8A is a block diagram of a safety circuit of a conventional dielectric barrier discharge lamp lighting apparatus. In FIG. 8A, the dielectric barrier discharge lamp lighting apparatus includes a dielectric barrier discharge lamp 101, a direct-current power supply 102, an inverter circuit 103 for converting the direct-current voltage of the direct-current power supply 102 to an alternating-current voltage, a step-up transformer 104 for boosting the alternating-current voltage from the inverter circuit 103, a drive circuit 105 for driving switch elements included in the inverter circuit 103, a voltage detecting circuit 106 for detecting a waveform of the high voltage output from the step-up transformer 104, and a comparator 107 for comparing shape of the detected high voltage output waveform with shape of the voltage output waveform in a normal lighting mode.
The dielectric barrier discharge lamp 101 has a discharge tube of 150 mm in length and 3 mm in outside diameter which is filled with 13.3 kPa of xenon gas as discharge gas, and an internal electrode of Ni bar is sealed at one end of the discharge tube. As an external electrode, a 0.5 mm Ni conductor wire is wound around the discharge tube. The inner wall of the discharge tube is coated with phosphor prepared appropriately in RGB colors in order to obtain desired light. The direct-current power supply 102 is, for example, a battery or chopper type direct-current power supply for producing a direct-current voltage of 24 V. The inverter circuit 103 has, for example, a configuration of half bridge type, full bridge type, or push-pull type, and turns on or off the switch elements included in the inverter circuit 103 by a signal from the drive circuit 105 to convert the direct-current voltage from the direct-current power supply 102 to an approximate rectangular wave alternating-current of, for example, 20 kHz. The step-up transformer 104 boosts the approximate rectangular wave alternating-current voltage from the inverter circuit 103, and converts it into an approximate rectangular wave voltage including ringing of high voltage of, for example, 3 kVp-p. The output voltage from the step-up transformer 104 is applied between the internal electrode and external electrode of the dielectric barrier discharge lamp 101 through a lead wire. The drive circuit 105 is formed of an exclusive IC or microcomputer, and controls the entire ballast circuit. The voltage detecting circuit 106 which is composed of resistors and others divides the output voltage of the step-up transformer 104, and detects the waveform. The comparator 107 compares the voltage waveform detected by the voltage detecting circuit 106 with the reference waveform, and sends a signal for stopping the operation of the ballast circuit to the drive circuit 105 if the waveform is changed more than specified amount.
The operation of such conventional dielectric barrier discharge lamp lighting apparatus is explained. When the power supply (not shown) is turned on, an approximate rectangular wave voltage including high voltage ringing is generated from the step-up transformer 104. The rectangular wave voltage of high voltage applied between the internal electrode and external electrode of the dielectric barrier discharge lamp 101 generates discharge in the discharge tube. When the discharge starts, xenon gas generates ultraviolet ray of 172 nm by excimer light emission. The generated ultraviolet ray is converted into a visible light by the phosphor of the inner wall of the discharge tube to render the dielectric barrier discharge lamp 101 emit light. At this time, since the dielectric barrier discharge lamp 101 operates normally, the output voltage waveform from the step-up transformer 104 becomes as shown in FIG. 8B. The voltage detecting circuit 106 outputs a signal proportional to the voltage waveform shown in FIG. 8B to the comparator 107. The comparator 107 compares the signal from the voltage detecting circuit 106 with a predetermined reference waveform signal. When the comparator 107 judges that normal lighting is done, the ballast circuit continues to light the dielectric barrier discharge lamp 101 without outputting a signal to instruct stop of operation of the ballast circuit.
On the other hand, if there is a trouble such as leak in the dielectric barrier discharge lamp 101, the dielectric barrier discharge lamp 101 does not emit light. Then the output voltage waveform from the step-up transformer 104 becomes as shown in FIG. 8C. The voltage detecting circuit 106 outputs a signal proportional to the voltage waveform shown in FIG. 8C to the comparator 107. The comparator 107 judges that the output voltage waveform from the step-up transformer 104 is extremely different from the normal voltage waveform shown in FIG. 8B based on the signal from the voltage detecting circuit 106, and sends a signal for instructing stop of operation of the ballast circuit to the drive circuit 105. The drive circuit 105 stops the output signal to the inverter circuit 103 based on the signal from the comparator 107, thereby stopping the operation of the ballast circuit.
Generally, since in the dielectric barrier discharge lamp, load characteristic is of positive capacitive characteristic, plural lamps can be operated in parallel by one ballast circuit. By contrast, in other discharge lamps than the dielectric barrier discharge lamp, such as a heat cathode fluorescent lamp or HID lamp, usually, one ballast circuit is needed for each lamp. Accordingly, in the ballast circuit of other lamps than the dielectric barrier discharge lamp, it is relatively easy to detect abnormality of lamp by detection of lamp current or the like. In a dielectric barrier discharge lamp lighting apparatus disclosed in patent document 1, also, one ballast circuit lights one lamp and copes with trouble of one lamp.
Patent document 2 discloses an example of safety circuit for detecting lighting failure of one or several lamps when a plurality of dielectric barrier discharge lamps are lit in parallel by one ballast circuit. According to patent document 2, in a discharge lamp lighting system including a ballast circuit connected to a plurality of dielectric barrier discharge lamps (referred to as “EEFL lamps”) of External Electrode Fluorescent Lamp type, a lighting sensor is provided in each one of the EEFL lamps, and an abnormal EEFL lamp is detected according to the signal from each lighting sensor operable to stop the operation of the ballast circuit.
Patent document 1: JP-A-2003-347082
Patent document 2: JP-A-2005-174909